So it looks like Nintendo WFC is still around, albeit in a limited and unofficial fashion. A group of intrepid Nintendo fans have set up a server that responds to many WFC requests, and is allowing some games to still go online. The system works by redirecting traffic from Nintendo's now defunct servers to a homebrew one set up by GitHub user Polaris, and handles matchmaking, online chat, and many other features that made Nintendo's first online venture the fun place it was.
For those of you who prefered Mario Kart DS's drifting style, or, I don't know, maybe still want to play Smash Bros, this server will allow you to (in many cases) continue playing as though Nintendo hadn't discontinued its online service. There is some work behind setting it up though, and for Wii players, you'll need to get homebrew apps working, but come on, haven't we all done that by now? (I haven't...)
The list of compatible titles is continuing to grow, so if you don't see what you like, be sure to follow the project and keep checking up on it. More user interest will hopefully keep our pals working on it!